1. Field of the Invention
The present invention relates to an electrode plate for a secondary battery with a nonaqueous electrolyte (which may be referred to as xe2x80x9celectrode platexe2x80x9d hereinafter) represented by a lithium ion secondary battery, and also relates to a process for producing the electrode plate. More particularly, the present invention relates to an electrode plate for a secondary battery with a nonaqueous electrolyte and a producing process thereof providing improved precisions in dimensions and thicknesses of an active material layer and a non-coated portion.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic equipment and communication equipment has been rapidly advanced, and this advance has been also required reduction in size and weight of secondary batteries used as driving power sources for these equipments. For these requests, there have been proposed secondary batteries with a nonaqueous electrolyte having high energy density and high voltage, typically, a lithium ion secondary battery, in place of conventional alkaline batteries.
For both positive and negative electrode plates which give great influence on performance of the secondary battery, it is desired, in order to elongate a charge/discharge cycle life and to realize a high energy density, to make larger an area of the electrode plate disposed in the battery by making thin the electrode plate.
For examples, Japanese Patent Laid-open Publication Nos. SHO 63-10456 and HEI 3-285262 disclose positive electrode plates which are produced by the steps of: dispersing or dissolving an active material powder for the positive electrode plate, which is composed of metallic oxides, sulfides, halides or the like, a conductive material and a binding material (binder) into a suitable wetting agent (referred to as xe2x80x9csolventxe2x80x9d hereinafter) to prepare an active material coating solution in a form of paste; and applying the active material coating solution on a surface of a collector as a substrate made of a metallic foil. In the electrode plate thus produced, as the binder, there is used fluororesin such as polyvinylidene fluoride or the like, siliconeacrylic copolymer, or styrene-butadiene copolymer.
On the other hand, a negative electrode plate is produced by adding a material, prepared by dissolving a binder into a suitable wetting agent (solvent), to an active material for the negative electrode such as carbon so as to prepare a coating solution for the active material in the form of paste and applying such active material coating solution on a surface of a collector made of metallic foil. Further, in order to improve the density of a coating film with respect to the collector and improve an adhesive property (adhesion) of the coating film thereto, a press treatment is usually performed.
It is required for the binder for preparing the active material coating solution for the above-mentioned coating type electrode plate to be chemically stable to the nonaqueous electrolyte, to be insoluble against the nonaqueous electrolyte and to be capable of being dissolved by some solvent and being coated thin on the substrate.
Furthermore, it is also required for the active material layer, which is coated and dried to have a sufficient flexibility so as to prevent peeling, chipping, cracking or the like at the assembling process of the battery and to have an excellent adhesion to the collector.
Further, terminals for taking out electric current are mounted to the positive and negative electrode plates, respectively, and both the electrode plates are then wound up together with a separator disposed between them for preventing short-circuit between both the electrode plates, which are then sealed in a container filled up with the nonaqueous electrolyte, thus assembling a secondary battery. In the thus assembled secondary battery, in the case of no capacity balance between the positive and the negative electrodes, there is a fear of causing various problems. For example, in a case where the negative electrode has a less amount of active material and the battery capacity of the negative electrode is smaller than that of the positive electrode, it is not possible to charge, into a space between carbon layers of the negative electrode, all the lithium ions coming out from the positive electrode into the electrolyte at the charging reaction time, and the lithium ions becomes excessive in the electrolyte, which forms lithium metal therein and which may deposit a dendrite (column-shape) on the negative electrode plate. If such deposit grows, the separator interposed between both the electrode plates may be broken, which results in the short-circuit therebetween, and the performance of the battery may be extremely damaged. In order to obviate such problem, the active material for the negative electrode plate is coated much in amount than that for the positive electrode thereby to keep the capacity balance of the positive and the negative electrodes.
Further, the electrode plates are usually provided with non-coated portions to which the active material is not formed such as portions to which the terminals for taking out electric current are mounted and boundary portions between the adjacent active material layers. A pattern of a non-coated (non-coating) portion is optionally determined in accordance with a design of the battery. The methods for preparing such non-coated portion include a method of directly forming a pattern of the coated portion and the non-coated portion by coating the electrode coating solution on the collector while mechanically controlling a coater head and a method of forming a coating film (coated film) entirely on the collector surface and then locally peeling the coated film by mechanical means such as knife thereby to form a non-coated portion.
In the former method, supply-start and supply-stop of the active material coating solution from the coater head are repeated while moving the coater head and/or collector in conformity with the pattern of the coated portion or non-coated portion, or every time the coating working reaches to the boundary portion between the coated portion and the non-coated portion, movement-stop and movement-restart of the coater head and/or collector, separation and re-approach of the coater head with respect to the surface to be coated, and supply-stop and supply-restart of the electrode coating solution are performed repeatedly in a synchronous manner. Intermittent coating workings are thus performed by the mechanical control of the coater head thereby to form an intermediate product of the electrode in which, for example, active material layers each having a length of 600 mm and non-coated portions each having a length of 50 mm are formed alternately repeatedly on the surface of an elongated collector having a predetermined width.
However, as the coating speed is increased, it becomes difficult to perform the mechanical control of the coater head so as to accord with the coating speed, and it becomes impossible to exactly form the pattern in which the coated portions and the non-coated portions are alternately repeatedly formed. Particularly, in a case where it is desired to form the non-coated portions, each having a relatively narrow area, intermittently repeatedly in the portion to be coated, it is extremely difficult to exactly form a pattern of the non-coated portion at a high speed. Furthermore, for the reason that the motion of the coater head cannot follow the high coating speed, local coating amount becomes slightly excessive at the respective supply-start positions, which results in the formation of a built-up (protruded) edge portion of the active material layer. On the other hand, there is a tendency of local coating amount being slightly short at the respective supply-stop positions, and so-called, tailing phenomenon will be caused, and hence, the edge portion of the active material layer inclines. In this inclined portion, the thickness of the active material layer decreases towards the boundary portion of the non-coated portion. The tailing phenomenon becomes remarkable as the coating speed increases and the inclined portion of the active material layer becomes long. When the tailing phenomenon becomes remarkable, the boundary line of the edge portion of the active material layer provides a wave-shape, thus being inconvenient.
As mentioned above, in the method of mechanically controlling the coater head, when the coating speed is increased, the shape and thickness of the edge at the peripheral portion of the active material layer is made in uniform. Then, if the capacity balance between the positive electrode and the negative electrode is determined in the assumption of dispersion at the peripheral edge portion of the active material for the positive electrode, much amount of the active material for the negative electrode is required, thus increasing material loss, and the battery capacity is made small in spite of the much using amount of the active material.
In the case where the edge portion of the active material layer is built up, a damage will be applied to the electrode plate and a pressing machine at the press working time to the electrode, it will become difficult to finely wind up the electrode plate and, moreover, the separator will be likely broken in the battery, thus providing problems.
In the case of less patterning performance of the active material layer, there will be provided a problem that the automatic sensing of the positions of the active material layer and the non-coated portion is made difficult. In the case where the active material layers are formed to both the surfaces of the collector through the mechanical controlling of the coater head, the active material layer having a predetermined pattern is first formed to one surface and, thereafter, another active material layer pattern is formed to the other surface while detecting the position of the first formed active material layer by means of sensor. Accordingly, in a case where the first mentioned active material layer is formed with a worse patterning performance, a patterning performance of the active material layer formed to the latter mentioned surface will also become worse. Furthermore, at the battery assembling time, the position of the non-coated position is automatically sensed, and in this time, in the case of worse patterning performance of the active material layer, this automatic sensing becomes itself difficult.
In the method of mechanically controlling the coater head, if the coating is carried out at a relatively slow coating speed, the above-mentioned inconveniences will be reduced to some extent. However, such reduction of the inconveniencies has a limit in its improvement, and moreover, it is difficult to increase the productivity of the electrode plates. As a method of performing the coating with a desired pattern, the coater head is mechanically controlled. For example, there are known methods of slot die coating, slide die coating and comma reverse coating, and except for these methods, there are also known, as methods which do not mechanically control the coater head, gravure coating and gravure reverse coating, for example. However, the methods not mechanically controlling the coater head are applicable to cases where a thin coating layer is formed, but not applicable to cases where a relatively thick layer such as active material layer is formed. For the reasons mentioned above, in the conventional technology, the active material layer and the non-coated portion are formed by the coating method in which the coater head is mechanically controlled in spite of less productivity of the electrode plate.
On the other hand, in the case of the latter mentioned method, that is, in the method in which the coated film is partially peeled by mechanical means such as knife after the formation of the coating film on the entire surface of the collector, the patterning performance is not made high and it is difficult to make smooth the edge portions of the active material layer, so that production of powder at the edge portion is caused, thus providing problems.
The present invention was achieved in consideration of the above facts and matters. The first object of the present invention is to provide an electrode plate having a high thickness precision at a peripheral portion of an active material layer and/or positional precision at a boundary portion between the active material layer and a non-coated portion.
A second object of the present invention is to provide a method of manufacturing an electrode plate having the high dimensional precision or high thickness precision at the peripheral edge portion of the active material layer.
According to the present invention, the following first to fourth electrode plates are provided.
(1) First Electrode Plate
An electrode plate for a secondary battery with a nonaqueous electrolyte at least comprising a collector, an active material layer containing at least an active material and a binder and formed in a predetermined pattern on the collector, and a non-coated portion at which the collector is exposed in a pattern complementary to the pattern of the active material layer,
wherein a maximum thickness of a peripheral edge portion of the active material layer in an area inside by 20 mm from a boundary portion between the non-coated portion and the active material layer is not more than sum of an average thickness of the active material layer and 10 xcexcm.
(2) Second Electrode Plate
An electrode plate for a secondary battery with a nonaqueous electrolyte at least comprising a collector, an active material layer containing at least an active material and a binder and formed in a predetermined pattern on the collector, and a non-coated portion at which the collector is exposed in a pattern complementary to the pattern of the active material layer,
wherein, in an inclined portion of a peripheral edge portion of the active material layer in which a thickness of the active material layer increase towards inside from a boundary portion between the non-coated portion and the active material layer, a width of an area, in which a thickness of the active material layer is not less than 1 xcexcm and less than an average thickness of the active material layer, is not more than 1 mm.
(3) Third Electrode Plate
An electrode plate for a secondary battery with a nonaqueous electrolyte at least comprising a collector, an active material layer containing at least an active material and a binder and formed in a predetermined pattern on the collector, and a non-coated portion at which the collector is exposed in a pattern complementary to the pattern of the active material layer,
wherein, an absolute value of a shifting of a boundary line between an actually formed active material layer and an actually formed non-coated portion with respect to a true boundary line of the predetermined pattern is not more than 1 mm.
(4) Fourth Electrode Plate
An electrode plate for a secondary battery with a nonaqueous electrolyte at least comprising a collector, active material layers containing at least an active material and a binder and formed in predetermined patterns on both surfaces of the collector, and non-coated portions at which both the surfaces of the collector are exposed in patterns complementary to the patterns of the active material layers,
wherein both the active material layers formed on both the surfaces of the collector are formed in plane symmetry with the collector being interposed therebetween and an absolute value of a positional shifting between a boundary line between an actually formed active material layer formed on a front surface of the collector and an actually formed non-coated portion on the front surface of the collector and a boundary line between an actually formed active material layer formed on a back surface of the collector and an actually formed non-coated portion on the back surface of the collector is not more than 1 mm.
The electrode plates according to the present invention satisfy at least one, preferably all, of the above conditions. Accordingly, the very high positional precision and dimensional precision of the active material layer and the non-coated portion can be achieved.
A preferred method of producing the electrode plates of the present invention mentioned above comprises:
a step for forming a high polymer resin layer from a high polymer resin selected from the group consisting of styreneacrylonitrile, polymethylmethacrylate, polydiisopropylfumarate and derivatives thereof in an area of a surface of a collector in which a non-coated portion is to be formed;
a step for forming an active material layer, by applying a coating solution for the active material layer containing at least an active material and a binder, on the collector surface on which the high polymer resin layer is formed;
a step for selectively performing thermo-compression bonding of a thermoplastic resin sheet or thermoplastic resin product to an area in which the non-coated portion is to be formed; and
a step for forming the non-coated portion at which the collector surface is exposed and the active material layer having a pattern complementary to a pattern of the non-coated portion by peeling off, after the thermo-compression bonding, the thermoplastic resin sheet or thermoplastic resin product from the collector thereby to peel off the active material layer, together with the high polymer resin layer, in the area in which the non-coated portion is to be formed.
In the method of the present invention, the coating solution for the active material layer is coated after forming the high polymer resin layer formed of the material mentioned above to the area of the surface of the collector in which the non-coated portion is to be formed. For the time being after the coating, the coated layer of the coating solution for the active material layer has not been dried and the high polymer resin layer is gradually dissolved and transferred in the coated layer. For this reason, when the coated layer has been dried and formed the active material layer, the high polymer resin layer is impregnated and solidified only in the active material layer in the area in which the non-coated portion is to be formed, and hence, the cohesive force of the active material layer in this area is made higher in comparison with that of the surrounding portion. Further, the high polymer resin layer has not been completely absorbed and somewhat has remain and exist between the collector and the dried active material layer in the area in which the non-coated portion is to be formed. For this reason, the active material layer in the area in which the non-coated portion is to be formed adheres to the collector through the high polymer resin layer having a weak adhesion.
Then, when the thermoplastic resin sheet or thermoplastic resin product is thermo-compressively bonded to the active material layer in the area in which the non-coated portion is to be formed, the thermoplastic resin sheet or thermoplastic resin product is secured to the active material layer in this area, and the thermoplastic resin layer is impregnated and then solidified in the active material layer in this area, thus further increasing the cohesive force.
Under this state, when the thermoplastic resin sheet or thermoplastic resin product is peeled and removed, the active material layer and the high polymer resin layer in the area in which the non-coated portion is to be formed adhere to the thermoplastic resin sheet or thermoplastic resin product and are then removed together therewith. In this manner, the non-coated portion at which the collector surface is exposed and the active material layer, which has a pattern complementary to a pattern of that non-coated portion, can be exactly formed.
Furthermore, in the method of the present invention, the non-coated portion is formed by forming the active material layer in form of the solid pattern on the collector and then peeling off the active material layer in conformity with the predetermined pattern. Accordingly, any build-up portion and/or long inclined portion is not formed at the edge portion of the active material layer.
Still furthermore, since the coating solution for the high polymer resin can sufficiently realize its function even with a small amount thereof, the coating solution can be coated on the surface of the collector by the method requiring no mechanical control of the coater head such as gravure coating method, gravure reverse coating method or the like method. Therefore, in the case where the coating solution for the high polymer resin layer is coated in the form of the predetermined pattern, this coating can be done at a speed higher than the coating speed for applying the coating solution for the active material layer in the form of the predetermined pattern with high precision and at high productivity. Still furthermore, in the case where the active material layer is peeled off by the method of the present invention, since the edge portion of the active material layer can be smoothly formed, production of powder at the edge portion of the active material layer is hardly observed different from the case of using a knife or the like.
The powder material may be dispersed in the high polymer resin layer. When the high polymer resin layers are formed on both the surfaces of the collector and the collectors are overlapped, a blocking phenomenon is liable to be caused. On the other hand, the dispersion of the powder material into the high polymer resin layer functions to prevent the blocking from causing and to impart a sliding property. Furthermore, the addition of the powder material will reduce the adhesion between the collector and the active material layer, thus being effective.
In one preferred embodiment of the electrode plate producing method of the present invention, it may be possible to selectively coat the adhesive, before the coating of the coating solution for the active material layer, in the area of the surface of the collector in which the active material layer is to be formed thereby to form an adhesive layer. In the case where the the active material layer is formed after the high polymer resin layer is formed to the area in the surface of the collector in which the non-coated portion is to be formed and the adhesive layer is then formed in the area in which the pattern of the active material layer is to be formed, the difference, in the peelability or adhesion property of the active material layer formed in the area in which the non-coated portion is to be formed, from that of the active material layer formed in the area in which the active material layer is to remain can be further made large. Accordingly, the selective peeling of the active material layer can be further easily performed.
In another one electrode plate producing method of the present invention, the patterns of the active material layers, arranged in a plane symmetry with the collector being interposed therebetween and having very slight positional shifting, can be formed on both the surfaces of the collector by arranging the areas in which the non-elected portions are to be formed to both the surfaces of the collector in the plane symmetry with the collector being interposed therebetween, forming the high polymer resin layers to the areas respectively, forming the active material layers to both the surfaces of the collector on which the high polymer resin layers are formed, thermocompressively bonding the thermoplastic resin sheets or thermoplastic resin products simultaneously to the areas of both the surfaces of the collector in which the non-coated portions are to be formed, and after the thermocompression bonding, the thermoplastic resin sheets or thermoplastic resin products are peeled off from the collector.
In this embodiment, after the active material layers have been formed on both the surfaces of the collector, the areas in which the non-coated portions are to be formed are simultaneously aligned in positions by clamping these areas by the paired thermoplastic resin sheets or thermoplastic resin products, so that the position alignment can be effectively performed and the positional shifting of the patterns on both the surfaces of the collector can be effectively prevented from causing. According to this embodiment, for example, the active material layer can be formed so as to have a pattern such that the absolute value of the positional shifting between a boundary line between the actually formed active material layer and non-coated portion formed on the front surface of the collector and a boundary line between the actually formed active material layer and non-coated portion formed on the back surface of the collector is not more than 1 mm.